Structure and Composition of an Egg

Eggs are one of the most nutritious and versatile foods in the kitchen are served on their own, used as an ingredient in many dishes starting from soup to desserts. It provides texture, structure, flavour and moisture as well as nutrition. Eggs can be brown or white; colour has no effect on quality or flavour but depends on the breed of the hen.

The egg is composed of Shell, Membrane, Aircell, Egg White, Egg Yolk and Chalaza. The egg white forms 2/3rd of the whole egg and the yolk forms 1/3rd.

1. Shell

• The Shell our outer covering of the egg, is either white or brown. 

• It is the outer hard covering of the egg and is made up of Calcium, Magnesium carbonate and Calcium phosphate.

• The shell gives shape to the egg and holds the inner contents.

• The shell contains thousands of pores that allow CO2 and moisture to escape, as well as air to enter.

• The shell is covered by a cuticle membrane or Bloom and should not be washed.

• The bloom acts as protective covering blocking the pores and prevents moisture loss and bacterial contamination.

• When eggs are washed before going to the market, the cuticle is removed.

• To protect the egg, the washed eggs are coated with a thin film of edible oil.

• The colour of the shell has no effect on the quality, cooking properties or nutritive value of the egg, the colour of the shell is normally determined by the breed of chicken. 

2. Membrane

• A membrane lines the shell and forms an air cell at the large end of the egg.

• The mother nature has worked very hard to create the right package for the contents of the egg.

• Beneath the shell, there are two semi-permeable membranes - the outer and the inner.

• These membranes act as a protective layer in case the shell cracks.

3. Aircell

• On one side of the egg (broader), both these membranes separate to form an air cell.

• This is formed by contraction of the contents as soon as the egg is laid, due to the difference in the outside temperature.

4. Egg white

• It has 1/8th part of the protein, which is called albumin; the remaining being water.

• The egg white consists of three parts – the outer thin albumen, the middle thick albumin and the inner thin albumin.

• The white is clear and soluble when raw and white and firm when coagulated.

• Albumin in the egg white is valued by the chefs for its ability to hold air when beaten.

5. Egg yolk

• The yolk is the unfertilized embryo n the egg, yellow in colour.

• The yolk is separated from the white by a membrane called the vitelline membrane.

• This membrane prevents the mix of both yolk and white. 1/6th parts of the egg yolk contain proteins, 1/3rd fat and the rest water, Vitamins and minerals like Calcium, Phosphorus, Iron etc.

• The yolk is held centred in the egg by the Chalazae and these are the two white strands which are present when the egg is broken.

• The yolk is high in fat and protein and contains iron.

• A Large yolk has around 59 calories.

• The yolk is valued for the richness and texture which it provides in both booking and baking.

6. Chalaza

• The egg is kept in position at the centre of the egg with the help of the chalaza.

• It has a thick-cord like an appearance and is composed of proteins.

• This chord-like structure may have to be strained while making custards.

• The function of the chalazae is to hold the yolk in place.

• In baking, the chalazae are sometimes removed in order to ensure a uniform texture.

Composition of egg

Nutritional Value of Egg:

The nutritional value of eggs varies with their size; it is not an important factor in judging their quality. Larger the eggs, of course, have more food value than small ones. A single large egg provides 6.5grm of protein or about 13% of the recommended daily intake for adults, as well as 80 calories and a good amount of iron, phosphorus, thiamine and vitamins like A, D, E and K. The disadvantage of the egg as a staple diet is their high cholesterol content.

The yolk of an egg which is about 50% water, 34% lipids fats and related substances and 16% protein with traces of glucose and minerals. The egg is made up of approximately 11% shell and 89% interior. The composition of the shell is important from the viewpoint of food safety, sanitation, and aesthetics. It contains calcium, carbonate (94%), magnesium carbonate (1%), calcium phosphate (1%), and 4% organic matter. It is important to recognize that there has been considerable information that hen's diet can impact the composition of the egg.